The chromosomal aberration rates (including gaps and breaks) and expression frequency of fragile sites were determined in peripheral blood lymphocytes cultured with TC 199 medium from 8 patients with squamous cell lung cancer, 10 of their first-degree relatives, and 12 healthy control subjects. As a result of cytogenetic evaluation, both the chromosomal aberration rates and expression frequencies of common fragile sites observed in patients and their relatives were significantly higher than those in healthy control subjects. Our results showed that common fragile sites might be unstable factors in the human genome, and their expression might be affected by some genetic and environmental factors. As a result of this they might play an important role in genetic predisposition to lung cancer. The high expression of fra(3)(p14) in patients and their relatives may be a valid marker for genetic predisposition to lung cancer. (C) Elsevier Science Inc., 1997.